Cellular communications systems often require that the radiated power from the transmitter be tightly controlled. Controlling the transmit power levels has several advantages. First, control allows minimizing the transmitted power which allows the overall power consumed by the cellular radio can be reduced. This reduction extends battery life which is an important metric in design of cellular handsets. Power control is also required for closed loop power control between the base station and the mobile device for system functionality in Code Division Multiple Access (CDMA) systems, as necessitated by the protocol used.
Controlling the transmitted power also has the affect of reducing the interference level in a cellular system. Often the total number of users in a cellular system is interference limited. In an interference-limited system the performance of any given communications link is limited by the interference generated by the transmitted signals of the many other communications links in use in the cellular system. If all transmitted signals in the cellular system are controlled to more accurately transmit at appropriate levels necessary to establish a given quality of service, then the total interference will also be minimized. By minimizing the total interference level generated by each communications link, the total number of communications links can be increased. However, increased accuracy typically cannot occur without increased cost.
In general, tightly controlling the transmitted power level is not trivial. One technique commonly used to control the transmit power is to use a digital multiplier to adjust the transmit power level while the signal is still represented digitally. The digital multiplier allows very precise control of transmit power and is very economical in both silicon area and power consumption. However, because the digital signal must be converted to an analog signal before transmission, attenuation occurs before a digital to analog conversion. Digital to analog converters usually have a limited dynamic range and extending the dynamic range of the converters is expensive. By placing a digital multiplier before the digital to analog converter, the dynamic range of the converter must be increased by at least the amount of the desired control range of the transmit signal. The increase in dynamic range of the digital to analog converter is, therefore, usually expensive and increases the power consumption of the digital to analog converter to a prohibitive extent.
A second method commonly used to vary transmit power levels is the use of voltage controlled amplifiers (VCA) in the RF portion of the radio. VCAs are placed after the digital to analog converters and therefore do not affect the required dynamic range of these devices. However, VCA's require a great deal of power to operate at high frequencies and occupy a large area.
A third method employed to control transmit power involves analog step attenuators. Analog “baseband” step attenuators have low power drain and can be realized in a small area. However, analog step attenuators tend to be inaccurate and do not allow the small steps or fine granularity in attenuation required by current cellular standards to meet transient adjacent channel leakage specifications.
Therefore, a need exists for a transmit power control system which simultaneously realizes small steps in attenuation, achieve great accuracy, consume little power, and occupies a small area.